Electrochemical capacitors, as intermediate energy storage systems between dielectric capacitors and batteries, have attracted much interest due to higher energy density than that of batteries and higher power density as compared to traditional dielectric capacitors. In particular, micro-supercapacitors are applicable as portable and lightweight power sources for miniaturized electronic devices such as micro-electromechanical systems (MEMS), micro-robots, wearable electronic textiles, and implantable medical devices.
Micro-supercapacitors can be coupled with micro-batteries or energy harvesting micro-systems to provide peak power. Generally, supercapacitor electrodes use nano- and micro-structured materials, which have the ability to permit easy access for electrolytes due to their high surface-to-volume ratios, instead of bulk materials.
There have been notable advances in the micro-supercapacitor field, which were mostly focused on enhancing energy and power densities through effective designs of different active materials, such as carbon nanotubes (CNTs), reduced graphene oxides, activated carbons, conducting polymers, and metal oxides.
For example, rolled-up structures of nanoscale-thick thin films have been utilized for energy storage systems. Schmidt et al. reported that the Swiss roll micro-supercapacitor based on RuO2 with length of 300 mm and diameter of ˜7 mm has the redox capacitance of ˜90 F/cm3 (˜2.2 mF/cm2) based on active electrodes in a three electrode system (Ji, H., Mei, Y. & Schmidt, O. G. Swiss roll nanomembranes with controlled proton diffusion as redox micro-supercapacitors. Chem. Commun. 46, 3881-3883 (2010)).
Micro-patterning technology has been developed for electrochemical capacitors with micro- or nano-scale thick deposition of active materials. Volumetric capacitance of ˜160 F/cm3 was obtained at ˜2 μm thick monolithic carbide-derived carbon films in 1M sulfuric acid, and the value was higher than that of ˜20 μm thick films (˜100 F/cm3).
Simon et al. reported that micro-supercapacitors produced by electrophoretic deposition of a several-micrometer-thick layer of onion-like carbon nanoparticles have powers per volume (˜300 W/cm3) that are comparable to electrolytic capacitors (Pech, D. et al. Ultrahigh-power micrometer-sized supercapacitors based on onion-like carbon. Nat. Nanotech. 5, 651-654 (2010)).
Ajayan et al. reported all-carbon, monolithic micro-supercapacitors with energy and power densities of ˜4.5×10−3 Wh/cm3 and ˜170 W/cm3 employing patterning and laser reduction of graphene oxide films (Gao, W. et al. Direct laser writing of micro-supercapacitors on hydrated graphite oxide films. Nat. Nanotech. 6, 496-500 (2011)).
However, most methods related to micro-patterning and rolling-up have been with MEMS technology, which is time consuming, limited in scale-up, and often causes the formation of cracks on metal substrates by deformation. Thus, there is a need to develop micro-supercapacitors that have high energy and power densities while maintaining their various shapes and energy retention performance even during deformation.